Grades of Carbon Fiber

Polyacrylonitrile Fiber

There is a lot of marketing techno-babble out there with all the different companies getting on the carbon bandwagon. The idea is to try and convince the general public that they are using something special by using technical terms that have little pertinent meaning. The real differences are fairly simple.

Almost all carbon fiber is made from a common industrial fiber called polyacrylonitrile fiber, also known as PAN. Most PAN fiber is used to make acrylic fiber. It is also used to make carbon fiber with a pyrolizing process, which means it is heated to ultra high temperatures to remove all elements except the carbon. Most carbon fiber is sold at this point and it has a tensile modulus of 33 million pounds per square inch (MSI). (Tensile modulus is a measurement of stiffness.) This 33 MSI fiber, if seen up close, looks like a a redwood tree trunk, with deep fissured bark. If processed further, the “bark” is stripped off, leaving a smoother round fiber that is smaller in diameter. More of these fibers can be packed into a smaller space, making it have a higher stiffness per cross sectional area. 42 MSI fiber is the result and it is informally known as Intermediate Modulus fiber or IM fiber. The benefit is that you can use less material to get the same stiffness and therefore a lighter structure.

Further processing can yield even higher stiffness fibers by making the fibers smaller and a little denser. These are fairly expensive, brittle and used sparingly. They are known as High Modulus fiber and are in the range of 55 MSI and higher. Many companies refer to 33 and 42 MSI fiber as “High Modulus” because they can’t get sued for false advertising by using this informal term. To understand the real grade of carbon fiber one needs to know the modulus of the fiber.

Fibers are bundled in various sizes designated in thousands (K) of fibers. 1K, 3K, 6K, 12K, 24K, 50K and others are common bundle sizes. These fibers are woven into fabric with various weave patterns. 3K fabric is most common. The various types of fiber will have the same “K” designation to indicate the number of fibers in the bundle. These numbers describe the size of the bundle used and have little to do with the quality of the fiber itself.

Carbon fiber is often sold pre-impregnated with epoxy and therefore is called “pre-preg”. Fabric can be pre-pregged but most pre-preg is unidirectional, meaning it has been spread out onto backing paper. It can be spread out in various thicknesses which is indicated with a GSM number which is simply Grams per Square Meter. This is very similar to how fiberglass is described using ounces, as in 12 oz. cloth, which weighs 12 oz. per square yard. Two layers of 6 oz. cloth are equal to one layer of 12 oz. cloth. The same is true for the various weights of carbon pre-preg. Again, these numbers do not have anything to do with the grade of fiber being used.

Of course there is more detail to describing carbon fiber, but the above describes the most important aspects.